In an IEEE 802.11 compliant communications system (also known as WiFi), an access point (AP) serves one or more stations (STA) by receiving transmissions from the one or more STA and forwarding the transmissions to their intended destinations. Similarly, the AP receives a transmission intended for one of its STA and forwards the transmission to the STA. A transmission occurs over unidirectional channels referred to as communications links. A transmission from a STA to the AP may be referred as an uplink (UL) transmission, while a transmission from the AP to a STA may be referred to as a downlink (DL) transmission.
In IEEE 802.11, the communication channel is shared by stations under a mechanism called distributed channel access with a function called distributed coordination function (DCF), which uses a carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. The DCF uses both physical and virtual carrier sense functions to determine the state of the communication channel. The physical carrier sense resides in the physical layer (PHY) and uses the energy detection or the preamble detection with frame length deferral when it determines that the communication channel is busy. In this disclosure the term preamble or the PHY preamble is used to identify that portion of the transmission at lower rate and usually generated at the PHY layer. The data part of the transmission it usually comes from the higher layers such as medium access control (MAC), Internet Protocol (IP), and/or session layers. The virtual carrier sense resides in the MAC and it uses reservation information carried in the Duration field of the MAC packet headers announcing the duration of packet transmission and packet acknowledgment over the communication channel. The virtual carrier sense mechanism is called the network allocation vector (NAV). The communication channel is determined to be idle only when both the physical and virtual carrier sense mechanisms indicate it to be so.
A STA with a data frame for transmission first performs a clear channel assessment (CCA) by sensing the communication channel for a fixed duration, commonly referred to as the DCF inter-frame space (DIFS). If the communication channel is busy, the station waits until the communication channel becomes idle, defers for a DIFS, and then waits for a further random backoff period. The backoff timer decreases by one for every idle slot and freezes when the communication channel is sensed busy. When the backoff timer reaches zero, the station starts data transmission. The communication channel access procedure is shown in FIG. 1.
IEEE 802.11 has recently launched the task group TGah, which is designed to support sensor/smart meters applications, as well as backhaul and cellular offloading for under 1 GHz. TGah will follow the IEEE 802.11 basic protocol with the support of 1 MHz, 2 MHz, 4 MHz, 8 MHz and 16 MHz channels, where only 1 MHz and 2 MHz channels are mandatory. In the actual specification framework document, the support for 1 MHz and 2 MHz channels is mandatory for all TG11ah compliant devices. Therefore the issues related to the coexistence between 1 MHz and 2 MHz (as well as other channel bandwidths) should be addressed.